A030 BCA detailed design of composite concrete bridge superstructures

Introduction The purpose of this publication is to extend the broad design intentions and considerations discussed in reference 1 in order to obtain the detailed design of various parts

DETAILED DESIGN

OF COMPOSITE CONCRETE BRIDGE SqdPERSTRQCTdRES

A Kumar BE, PhD, CEng, MICE, MIStructE

[= bending inertia of the top slab;

I = bending inertia of the bottom slab;

I, = bending inertia of the equivalent |

length of longitudinal beam about

the Z axis;

a = spacing of the longitudinal beams

b = distance between the neutral

axes of the top and bottom slabs

Acknowledgements

lam most grateful to Mr E M Jones of Cleveland County Council (formerly of Staffordshire County Council) and

Mr J F White of Staffordshire County Council for checking the design calculations and making many constructive

criticisms

I would like to thank many former colleagues at the Cement and Concrete Association* who have contributed

indirectly through their discussions with me, notably Mr Tony Threlfall | am also grateful to Dr Tom Harrison

and Dr Bill Cranston for their interest and provision of the necessary facilities at the Cement and Concrete

Association to carry out this work Finally | would like to thank Mr A E Brooks for his editorial contribution to

the text

Beaconsfield

First published 1988 British Cement Association

All advice or information from the British Cement Association is intended for those who will evaluate the significance and

limitations of its contents and take responsibility for its use and application, No liability (including that for negligence) for

any loss resulting from such advice or information is accepted Readers should note that all BCA publications are subject

to revision from time to time and should therefore ensure that they are in possession of the latest version

DETAILED DESIGN OF COMPOSITE

CONCRETE BRIDGE SũPERSTRäCTRES

A Kumar BE, PhD, CEng, MICE, MiStructE

Contents

Introduction © 0 ee cnet ee eee been bebe cece nnennes 2 Extent of design effort 20.2 0.0.02 eee eee eee beeen ee nestenas 2 Simply supported and continuous T-beam superstructures using M beams .0- 2 Inverted T-beam, solid infill superstructures 0.00000 cc ccc cece eee eee eet eeeeues 4 Pseudo-box superstructures with M beams QQQQ Q QQ Q eee e ee eennnenns 5 -beam superstructures 2.0.0.0 eee eee cence nnn e eee ennevennas 6 Box-beam superstructures 0 0 ccc ce ene en tee een ete neennnnaes 6 l-beam superstructures LG Q Q Q Q Q g0 Q TQ ng g n vn ng g ng g vn n kg v va ? S04 An nu en een e eee ee een eee ee ee nen eeneneennnnnns 7 Example A : Simply supported composite concrete superstructure 8 Example B: Continuous composite concrete bridge superstructure 49

Introduction

The purpose of this publication is to extend the broad

design intentions and considerations discussed in

reference 1 in order to obtain the detailed design of

various parts and components of composite concrete

bridge superstructures For obvious reasons, it is not

possible to cover the detailed design of all types of

beam and their possible applications However, two

examples, representative of practical bridging

situations and utilizing the most popular of the beam

types, have been chosen for detailed design here and

are discussed on this page

Since the various types of precast beam and various

requirements of the Code'**) have many structural

similarities, only the essential ways in which other

types of superstructure differ from M-beam superstruc-

ture design are outlined on pages 4 to 7 Full details

about the standardized sections, manufacture of

beams and preliminary superstructure design are con-

tained in reference 1

Extent of design effort

Since the initial publication of the limit state Code in

1978‘, many engineers have expressed the view that

the five new load combinations and the two basic limit

states would considerably increase the analysis and

design work required for bridge design Experience has

not, however, supported this view Combination 1

remains essentially the same as in the previous ap-

proach, i.e only permanent forces and live loads need

be considered in the design Combinations 2 to 5 in the

new Code”? clearly define how the various transient

forces are to be combined This contrasts with the

previous Combination 2, in which all compatible forces

had to be considered, and yet no guidance was given on

how this was to be done, thus engendering inconsisten-

cies in bridge design practice

Additionally, separate calculations were previously

required for reinforced concrete and prestressed con-

crete, to check against the permissible overstresses®°)

in the materials of up to 30% of the basic values when

HB load was present on the bridge deck In the new

Code! this is no longer the case, although some com-

plication is caused by the requirement to check crack-

ing in reinforced concrete and tensile stresses in

prestressed concrete under serviceability Combination

1 for 25 units of HB, compared with 45 or 37.5 units of

HB (depending upon the class of the road) forthe rest of

the design

The two detailed design examples included in this

publication are intended to illustrate that an adequate

design complying with the Code can be obtained with-

out either an excessive amount of computer analysis of

the superstructure or lengthy hand design calculations

If more precision is desired or the behaviour of the

superstructure is not well known, additional loading

cases for the computer analysis are recommended In

time, with the accumulation of experience with the new

2

Code, engineers will be able to discern the critical

loading conditions more readily and take short cuts, as they have done in the past, to arrive at satisfactory designs

Simply supported and continuous T-beam superstructures using

M beams

To illustrate the design procedure and compliance with

the Code, two representative examples of the cesign of

a composite concrete superstructure have been in-

cluded at the end of this publication Both examples utilize M beams in the form of a T-beam superstructure construction (there is no bottom-flange in situ con- crete) Example A deals with the design of a single- span simply supported superstructure using (a) fully bonded, (b) part debonded or (c) part deflected tendon configurations in the precast beams Example B deals with the design of a two-span live load continuity-type superstructure (as described in reference 1) using part deflected tendons in the precast beams

Example B has been extended to deal with the design of the continuity moments over the middle support in the design of the middle diaphragm The end diaphragms, the links in the precast beams and the top slab are also designed for this second case (design for the simply supported case will be broadly similar) References to various parts of BS 5400 and clause numbers are indicated in the left-hand margin of the examples for the benefit of the reader

For both these examples, substantial diaphragm members exist at the support positions which are shown to possess sufficient strength for separate bear- ings to be provided under alternate beams only During the construction period, each beam will obviously be provided with a separate temporary bearing until the diaphragms are cast

The Department of Transport have issued imple- mentation documents for the Code"®) and these have also been taken into account in the detailed design cal-

culations of Examples A and B Table 1 indicates the

values of Y, to be used in the calculations for these examples The section properties for the precast M beams are given in references 1 and 9 The section properties for the composite section using M beams

with 160 mm top slab are given in Table 2

In the context of live load continuity in composite

construction, the Code gives some guidance (Part 4,

Clause 7.4.3.5.) and formulae (Part 4, equations 33 to 35) for estimating continuity moments It should be noted that the hogging restraint moment due to differ- ential shrinkage, M,, given by equation 33, is only correct for a large number of spans made continuous

by this method For only two spans, this restraint mo- ment should be increased by 50%

Table 1 Loads to be taken in each combination with appropriate Y,,

deck surfacing (LS† 17500 061.75 1/75 -175 1.75

SI.ST 1.20 1.20 1.20 120 1.20

SLS 1.00 1.00 1.00 1.00 1.00 5.1.2.2 & Reduced load factor for dead and superimposed qLS 1.00 1.000 1.00 1.00 1.00 5.2.2.2 dead load where this has a more severe total effect

5.8 Earth pressure: retained fill and/or live load surcharge aLs 1.50 1.50 1.50 1.50 1.50

SLS 1.00 1.00 1.00 1.00 1.00

6.8.1 Vehicle collision load with bridge parapets and associated Sẽ

SLS | 36 1.20

6.8.2 for global effects with containment parapets: se

* 1, shall be increased to at least 1.10 and 1.20 for steel and concrete respectively to compensate for inaccuracies when dead loads are not accurately assessed

+ T, may be reduced to 1.2 and 1.0 for the ULS and SLS respectively subject to approval of the appropriate authority (see Clause 5.2.2.1 of BS 5400 : Part 2)

t This is the only secondary live load to be considered for foot/cycle track bridges

NOTE For loads arising from creep and shrinkage, or from welding and lack of fit, see Parts 3,4 and 5 of BS 5400, as appropriate.

Table 2 Section properties of composite M beams, beam spacing 1 m, modular ratio = 1.0

M2 850 467 650 429.3 42.98 102.16 147.84 164.86 100.11

M3 930 499 650 474.37 54.49 119.59 167.33 184.31 114.86 M4 1010 531 650 517.98 67.69 137.57 186.98 203.87 120.68 M5 1090 506 050 553.00 81.82 152.37 201.04 217.04 147.96 M6 1170 538 050 601.21 98.80 173.70 225.16 241.69 164.34 M7 1250 570 050 649.10 135.79 225.97 288.36 307.98 209.20 M8 1330 544 450 675.84 135.31 206.84 258.14 273.81 260.20 M9 1410 576 450 727.29 158.56 232.25 286.87 303.34 218.01

Restraint moments due to prestress and its profile,

and dead load, both modified by creep, should also be

calculated (no expressions are given in the Code) The

creep modification factor for these latter effects sug-

gested in the Code is given by equation 35 but assumes

that continuity is established at the time of prestressing

the beams This could result in seriously overestimat-

ing the support sagging moments, since much of the

prestress and dead-load creep would have taken place

prior to the establishment of continuity For this reason,

the approach used in Example B is based on ‘first

principles’ and only involves the residual creep due to

prestress and dead load at the time of establishing con-

tinuity Some information on the background of the

Code equations can be found in reference 10

Inverted T-beam, solid

infill superstructures

The live loading on an inverted T-beam superstructure

with solid infill concrete will be resisted by the rectangu-

lar composite section and this should be reflected both

in the analysis and in the design of the superstructure

The design of diaphragms and top slab will be replaced

by the design of transverse reinforcement through the

web holes for the whole length of the span If no

longitudinal bars (parallel to the beams) are placed at

the bottom of the in situ concrete between the beams,

the crack width for the transverse bars only needs to be

4

checked at a nominal cover away from the surface of these bars Longitudinal steel, if required at this posi-

tion, should be placed on either side of, not directly

above, the gap between the bottom flanges This will avoid contravening the crack-width limitations of the Code for these bars (see Figure 1)

As indicated elsewhere"), lightweight-aggregate concrete is sometimes used as an in situ infill medium with this form of construction in order to reduce the dead load of the structure The Code does not ceal with

the use of such concrete as prestressed members in

highway structures However, as an in situ infill me-

dium, this concrete is only reinforced in the transverse

direction and can be dealt with under the provisions of the Code In the longitudinal direction, the lightweight-

aggregate concrete acts compositely with the prestressed member

For stress analysis of the composite section for the

serviceability limit state, the lightweight part of the

section can be transformed on the basis of the modular

ratio of the two concretes According to the Code, the

modulus of elasticity of lightweight-aggregate concrete

is related to that of normal-weight concrete of the same

grade by the square of the ratio of their dry densities

For the flexural strength of the composite section at the

ultimate limit state, the modular ratio will not be appli-

cable, and the grades of two concretes will determine

their contribution to the compressive force in the sec-

tion Shear strength of composite sections can be

determined as for normal concrete superstructures,

albeit that the reduced Code values apply for the

lightweight component Alternatively, as for all forms

of composite construction, the precast units them-

selves can be designed to resist the entire shear force

on the composite sections, as indicated in the Code

For the analysis of the superstructure, the stiffness

values calculated for the individual members should be

based on the transformed concrete section in both di-

rections, to reflect the lower modulus of elasticity of the

lightweight-aggregate concrete

Pseudo-box

superstructures with

M beams

For pseudo-box superstructures with M beams, the live

load on the deck will be resisted by the pseudo-box

section, the properties of which should be considered in

the grillage analysis and may be calculated as follows

The longitudinal flexural inertia of the composite

section may include full contribution from the in situ

concrete on the bottom flanges, as shownin Figure 2"),

The transverse flexural inertia of each strip equal to the

spacing of the web holes may be calculated by replac-

ing the whole of the bottom in situ concrete and the rein-

forcement by an area of concrete which is 4D horizon-

tally and 2.5D vertically, where D is the diameter of a

single equivalent bar equal in area to that of the

transverse reinforcing bars through the hole This is

indicated in Figure 3"),

The longitudinal torsional inertia for a single cell

should be calculated using the thin-wall formula by

Transverse section

Figure 3 Section for transverse stiffness evaluation

considering half the thickness of the beam webs in con- junction with the top slab and the bottom in situ

concrete taken uniform of its maximum thickness,

which occurs mid-way between the bottom flanges as shown in Figure 42):

4A2

ý ds

t area inside the median line of the concrete; and

sum of the lengths of the sides around the median line each divided by the appropriate wall thickness

Torsional inertia, C = where A

equivalent closed box section and then applying the

thin wall formula

The thickness ¡ of the equivalent continuous side

wall for the idealization shown in Figure 5 is given by the

formulat1?):

where I = bending inertia of the top slab;

s ° bending inertia of the bottom slab;

I,, = bending inertia of the equivalent length of longitudinal beam about

the Z axis;

a = spacing of the longitudinal beams

b = distance between the neutral axes of the top and bottom slabs

The method of calculating torsional inertias in the two

directions for a pseudo-box, M -beam superstructure is

illustrated with the help of a numerical example in

Appendix 1 of reference 12

The superstructure is acting both ways simultane-

ously and the longitudinal torsional inertia calculation

is more likely to be an accurate assessment of the total

torsion stiffness of the superstructure The longitudinal

value should therefore be split into longitudinal and

transverse components in proportion to originally cal-

culated values in the two directions, for in-putting into a

grillage analysis program This procedure is discussed

by West in reference 12 The contribution to transverse

stiffness from the diaphragms should be included in the

support edge members of the grillage

The presence of the bottom in situ concrete is usually

~ Spacing of transverse grillage members

ments Crack control forthis bottom flange steel can be handled in the same way as for the inverted T-beam

superstructures, except that the simplified equation for

flange in over all tension, given in the Code, should be

slab, will be very substantial Also, unlike the T-beam

superstructure using M beams, the torsional liriks in the

webs can be spaced further apart because of “he good

proportions of the sides of these sections and the Code

requirements in this respect

The section properties forthe composite section with

CU beams and 160 mm top slab are givenin Table 3 The

unsupported webs of these beams are vulnerable to mishandling and shock loads; therefore, during the manufacture, lifting and transport of these beams, some temporary bracing of the webs is often necessary

to avoid overstressing the junction of the webs with the bottom flange

Box-beam superstructures

Box-beam superstructures are only provided with a

nominal thickness of levelling screed with mesh rein-

forcement; their transverse distribution properties emanate from post-tensioning through transverse dia-

phragms The precast box beams therefore carry the

entire longitudinal effects themselves, while in situ infill concrete (between the beams) combined with post- tensioning, handles the load distribution and the result- ing transverse effects Thus, any structural contribu-

tion from the screed or topping which may be provided

is ignored, both in the analysis and in the design of this

type of superstructure Vertical shear due to live loads

and longitudinal shear due to a component of trans- verse prestress in skew bridges (if prestress is applied parallel to the supported edges) should be checked at the vertical interface of the precast and the in situ

concretes

Table 3 Section properties of composite C beams; beam spacing 2 m, modular ratio = 1.0

dl 960 786 450 568.62 82.87 211,75 358.17 145.75

q5 1160 852 450 685.87 136.31 287.49 433.01 198.73 ũ7 1260 885 450 743.83 169.29 327.98 475.32 227.59

| - be a m 5 DEPARTMENT OF TRANSPORT Reinforced concrete

2 for highuuau structures London, 1973 BS 1/73

su p erstructures 6 DEPARTMENT OF TRANSPORT Prestressed concrete

for highway structures London, 1973 BS 2/73 This type of superstructure is essentially similar to the ;

T-beam construction using M beams, except that there @, DEPARTMENT OF TRANSPORT Loads for highway are in situ transverse diaphragms acting compositely bridges: use of BS 5400: Part 2: 1978 London with the top slab, which must be considered both in the BD 14/82: 1982 and its Amendment No 1 :

analysis and in the design of this type of superstructure 1983

8 DEPARTMENT OF TRANSPORT Design of concrete bridges: use of BS 5400: Part4: 1984 London

9 PRESTRESSED CONCRETE ASSOCIATION,

1 KUMAR, A Composite concrete bridge _ Prestressed concrete bridge beams Second

superstructures Wexham Springs, British edition Leicester, 1984 7 pp + folder

Cement Association, 1988 46 pp Publication

46.505 10 CLARK,L.A Concrete bridge design to BS 5400

London, Construction Press, 1983 186 pp

2, BRITISHSTANDARDS INSTITUTION Steel, concrete

and composite bridges Part 2: Specification for 11 MANTON, B.H and WILSON, C.B MoT/C&ECA

loads London 46 pp BS 5400 : Part 2:1978 standard bridge bears: prestressed inverted

T beams for spans from 15m to29m London,

3 BRITISH STANDARDS INSTITUTION Steel, concrete Cement and Concrete Association, 1975 20 pp and composite bridges Part 4: Code of practice for Publication 46.012

design of concrete bridges London 68 pp

BS 5400: Part 4: 1984 12 WEST,R CGCA/CIRIA Recommendations on the

4 BRITISH STANDARDS INSTITUTION Steel, concrete

and composite bridges Part 4 : Code of practice for

design of concrete bridges London 48 pp

BS 5400: Part 4: 1978

use of grillage analysis for slab and pseudo-slab bridge decks London, Cement and Concrete Association, 1973 24 pp Publication 46.017

EXAMPLE A: Simply supported composite

concrete superstructure

End support and điaphragm section .- eee ee tte teen ene 2 Deck section and grillage idealization .- -.- ee eee ete 3 Stiffnesses for grillage analysis - Examples A andB 2 et eee 5 Live load patterns 2.00 ee ee nee ee eee eee eee 7 Basic load cases for grillage analysis 2.0.0 cc eee eee eee ete hà ki mi ng 10 Longitudinal moments in precast beams 2.0 ee eee ee eee 11 Section properties .0 0.0 ee eee ee ee eee eee eens 14

Allowable stresses in precast concrete 1.2 0 cee ce ee ee ee eee eee ene 15

Stresses due to dead load and live load 1 2 Lee ee nee teen eens 16 Temperature difference stresses (PTD and NTD) ee eee nee 17 Differential shrinkage stresses (DSH) Q HQ HQ HH hy hy hà 20 (A) Precast beams with straight fully bonded tendons Ặ ẶQ Q HS 21 Differential creep stresses (DC) Q Q Q LH Q HQ HQ Q H ng HH n HH gà H ĐH nà kà kh kh va 25 Final stress checks Q Q Q HQ HQ HQ HH HH HH Ho gu Hy ng kg Hy kg k KV Hy kg eens 28 (B) Precast beams with straight part debonded tendons 29 (C) Precast beams with straight part deflected tendons -.-ẶVẶẶ VỤ 38 ULS checkS 2 ee ee ee ee ee ee ee ee 40

8

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2x6o + 6X llO + x!éO+ 2x 430 + 2x S550

€ - 37D - _— ,

23

= 810 — Ííô = /42 mm

e - v

transfer ano Anat: Shortening Ar tuUns

inital stemig due do dead ford and ÂytA #22

ef ma- pan are at Sharm om fg 1S /A

Fig IS/ Stresses a transfer art mid shan

22/1 A I

Drawing ref Calc by Date Check by Date

Project dob ref

Part of structure Calc sheet no rev

fib 2 = ~O8325x OSB 2 -O-4E ad

Ref Calculations Output

Lea of f etl Ar Fhe (eng (Fone:

t/6-722 Cu» va we fost x 0:02 (Say) ¥ 4002 = 8OkN

724 — Shninkare lots ( Eos Es Abs)

= 300x10 195%23%/39= IRTEN

Yenrs| Gap bors (6 foo ks ops)

+ z4 obmg “fo the Dt meme , te to

faas revll vary along Me bean, Wre fol

trké t2 thea tere buen daa ew -2/4A—

Mew conchbin xx Ac memder, “test Frese

be brad ew cv, sÊxae alan whe, Cops fe beau,

‘ ae oy ean owe § - awe a ae oe SR RY

Tair py nek gamail cam Le SP Ryas ALAA xŠ MAP DCSE Wet be

Project Job ref

Partof structure D/FFERENTIAL CREEP Calc sheet no rev

Ref lo

% Tạ 10

Basis af tale lation (See comments on Faae 3)

Asse YLat Aso - Wk As “f up ont +o

pr lirse ancl enon self magi Hake 7 pee frieg +o The coctimp of fp Hab( sory

ak 180 dogs ) powdfere the Aciiaduval eed

for bees = x GE The Lame neers

factor can fe Adcumed fer the slab ⁄

he flal clad Lond ord us afphcd

ot “ 160 Mays whew the beam conn i

cen aiclr.ably mre mats han al Rewmsfr

Assume tet tyo-tivels ef the total di

od preatres ag ti Coop vi k2 plac “ee

«4/46 n cook, tharspere Fey= Fat t(P-Fe)

Fesheus, C force dp rear nt xen ial ‹ ~ee/

Are Ýo kh‹Z +2 F p= — fe;.Poah

Eecenti city of prtcael- earn cerlisad to

the NI nước centrerd “¿¿= 7Ø - 3!0= 164 mm

Nlom wn abart Preceat tram tentreiad to

Pustornr bnrvadirr clre Có Afperentiol reef

= (Rye- My, ) Bees ?

Kelhinze re amd memint on the Compote

seetrer, witt therefore be — Fran of

Ay mid — spam a con fartooive strez of

O-4Y Nim” exists ot feel

wea

|

Ề

a7 iA |

Drawing ref Calc by Date Check by Date

Project Job ref

Partofstructure Fyng/ Stress Checks Calc sheet no rev

Project Job ref

Part of structure ( 6 ) PRECAST BEAMS WITH

STEM GHT PRET PEBENPtbD TENDONS

Calc sheet no rev

p2i/A

Assume 207, Lew of prusburr subacg nent fo

3, 337 _ s z

A ergy = 337 Nim at fins 1 and 2

ropectirely ak grantor, Sion powtens Tare fare

Allowing 107 Loss of preshrss Cefore amd during |

Tams fer ; ®e</me respec d Be 2980 LM

TAL 22 xaxrv2 at a stor of 075 Ton br ^<:pC 20 strandsI7! 7, f5:2 Mw, StandarA `

30, A |

Drawing ref Cale by Date Check by Date

to oblav~ Stine peri bilirs in hitondhing ) |

Fry the skand petierar of Fig 1e/A

Fig 18) SMa ol pettan with de bonded t-dons |

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